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Old 03-21-08, 02:17 PM   #1
kukusz
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Aero Benefits of Rear Wheel

I'm thinking of getting a set of carbon tubulars. All this nonsense about drag and dimples and rotational mass has deluded me into thinking I might be faster. Therefore, I ask you to humour me and not tell me crap like I won't be faster - I already know this. Consider this purely an academic discussion.

Does anyone know the added benefit of a rear aero wheel? Given the fact that the wheel is behind the frame and riders legs, is there really any noticeable difference? The reason I ask is because it is well within my budget to get a nice front and keep my Open Pro and Powertap rear. Sure, the thing will be ugly as hell, but I could care less. I don't really feel like getting a set just so it looks complete is worth it if the added benefit is like 10% of just the front wheel. Plus, I could always get the rear later.
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Old 03-21-08, 02:20 PM   #2
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i applaud you for being willing to ride a nice front wheel and a terrible unmatching rear!

Since so many people TT with rear discs there is obviously some advantage there.
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Old 03-21-08, 02:26 PM   #3
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i applaud you for being willing to ride a nice front wheel and a terrible unmatching rear!

Since so many people TT with rear discs there is obviously some advantage there.


More people would use front disks if they weren't so squirrelly.
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Old 03-21-08, 02:27 PM   #4
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Since so many people TT with rear discs there is obviously some advantage there.
I agree, however I could see that it takes a rear disc to get any real advantage, so just putting a 58mm on the back won't do jack.
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Old 03-21-08, 02:35 PM   #5
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Discs are more aero off the bike and with certain conditions. According to the HED gurus a deep sectioned wheel is equally as fast on a bike with a seat tube cutout for most conditions. Discs also don't have a huge benefit margin until yaw angles above 15 degrees.

Not sure if they wanted me to tell, so I'll leave the names out, but a couple of big name riders are going to be on H3D and Stinger 90s this year because they tested faster than the disc(on their particular bike and position).

As for percentages, I don't know, but the front wheel is definitely the one to get first. Maybe a disc cover in the back to offset the cost, but get a bunch of aero benefit too?
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Old 03-21-08, 03:09 PM   #6
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I should show up at my next crit with a 1080 and a rear disc. I wonder if the officials would even let me warm up...
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Old 03-21-08, 03:24 PM   #7
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I should show up at my next crit with a 1080 and a rear disc. I wonder if the officials would even let me warm up...
the 1080 is certainly legal. The rear disc I believe may also be legal as long as it is not an event which would have to abide by UCI rules. Still, you would get laughed at.

as to the OP, I remember reading the difference in drag from a slow 32 spoke box rim to a high end disc can range from sucking up 50 watts (for the slow wheel) and 0-10 watts for the rear disc. That is 40-50 watts of savings. Whether or not that is true remains another thing...I can't seem to find the web reference for that. Going to ANY type of aero will get you your biggest return (box rim to say a 58 mm). After that, there are diminishing returns so the 58 to a 108 mm rim will not give nearly as much help.
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Old 03-21-08, 04:41 PM   #8
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And for a $100 you can get a wheel cover cut to fit your rear wheel from Wheelbuilder dot com and get 98% of the aeroness of a disk wheel at a fraction of the cost.

Really, what reason not to?
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Old 03-21-08, 04:42 PM   #9
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Does anyone know the added benefit of a rear aero wheel?
They sound really cool when they go by, whoop whooop whoopp......

Its like a playing card and a clothes pin for adults.
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Old 03-21-08, 07:44 PM   #10
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They sound really cool when they go by, whoop whooop whoopp......

Its like a playing card and a clothes pin for adults.
Thats still cool.

I've also read that after the aero wheels 2nd best thing you can do is an aero helmet. True?

I ask 'cause my "a" goal this year is the district TT and would like to take advantage of everything i can within the rules.
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Old 03-21-08, 07:48 PM   #11
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According to MIT (I think?) the helmet will help more than the wheels, actually. Definitely heard that somewhere.
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Old 03-21-08, 07:53 PM   #12
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According to MIT (I think?) the helmet will help more than the wheels, actually. Definitely heard that somewhere.
Who wants to take this one? Beuller?
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Old 03-21-08, 08:24 PM   #13
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I'm thinking of getting a set of carbon tubulars. All this nonsense about drag and dimples and rotational mass has deluded me into thinking I might be faster. Therefore, I ask you to humour me and not tell me crap like I won't be faster - I already know this. Consider this purely an academic discussion.

Does anyone know the added benefit of a rear aero wheel? Given the fact that the wheel is behind the frame and riders legs, is there really any noticeable difference? The reason I ask is because it is well within my budget to get a nice front and keep my Open Pro and Powertap rear. Sure, the thing will be ugly as hell, but I could care less. I don't really feel like getting a set just so it looks complete is worth it if the added benefit is like 10% of just the front wheel. Plus, I could always get the rear later.
What you haven't mentioned is the, "What are you going to use it for?" Crits., TT, RR... Going tubular is a likely weight savings of tubular over clinchers. I just got an old set of Zipp tubular 404's. the savings of 1lb over my campy 28 hole 14-15 gage std. spoke -Front wheel alone-
Weight savings of the wheels pays every time you accelerate or climb, The aero saves watts in most every situation except a super tailwind.
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Old 03-21-08, 08:27 PM   #14
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Who wants to take this one? Beuller?

I will! ZeCanon is perfectly correct. And don't wear gloves.
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Old 03-21-08, 08:40 PM   #15
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According to MIT (I think?) the helmet will help more than the wheels, actually. Definitely heard that somewhere.
Here you go: http://www.bicycling.com/article/1,6...4995-1,00.html.

"Most people don't realize that a nonaero helmet creates four times the drag of a nonaero wheelset. So you can spend two thousand dollars on a wheelset, or spend two hundred on a helmet and be faster. How you put your race number on matters more than having an aero wheel; today, we glued on our numbers to get them to fit flatter. Then there's water bottle placement: On a round-tubed frame, having a bottle on your seat tube is more aerodynamic than not having one at all, and it's much more aero than putting it on the down tube. And wearing gloves in a time trial will slow you down more than using a nonaero front wheel."
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Old 03-21-08, 08:58 PM   #16
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If I might be so bold as to take the pseudo-quote the argument of Asgelle - "the MIT data is at only a zero-degree wind angle"- i.e., straight on... "and XX degrees (generally 15) is the predominate wind angle." My apologies to asgelle if I'm being presumptuous or have misinterpreted.

My argument: The TT's that matter to me have always been dead-wind. And even if they weren't, you'd have to prove to me that the predominate wind angles are in the X-XX degree angle. Even assuming that, you'd still have to prove to me - with person-on-bike data - that helmets are not more of an effect than wheels.
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Old 03-22-08, 05:56 AM   #17
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I should show up at my next crit with a 1080 and a rear disc. I wonder if the officials would even let me warm up...
I raced at Ninigret Park RI with a rear disc and a TriSpoke (HED3) up front. P123, I felt I had no chance since I was a 3, the course favors breaks (super windy - beach side airfield, 8 turns), and I can't TT to save my life.

I have to find a picture, I think I even put on my Rakes.

I told my friends/teammates that the 3rd break always wins so I'll go after that one. The first one went, never got a gap, came back. Second one went. Local strongman Amos and another local strongman Leigh. I said "F that 3rd break rule this one is going to work". I bridged up. Worked as hard as I could (i.e. I pulled 1x/lap - they were doing 2 pulls each).

Stayed away for 10 laps or so. Got caught, flurry of counter attacks, and TWO groups went up the road.

I haven't managed to stay in a race down there for more than 4-5 laps since then but the next time I go, I'm waiting for the 3rd break.

Regarding helmets I've considered starting races with a bob tail TT helmet (Lemond 89 Tour helmet) when it's cold. I tape my vents anyway. Giro sold a helmet with a ventless shell (92?) where they simply covered the then-high-number 9 vents with the shell. This was meant as a "road TT" helmet.

In the summer though I never get away and I'm always hot. So vents it is.

cdr
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Old 03-22-08, 06:39 AM   #18
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Actually I was thinking of this, http://forum.slowtwitch.com/gforum.c...helmet;#908055

and if anyone thinks Zipp is partial, you might want to read what MIT had to say about it. http://forum.slowtwitch.com/gforum.c...helmet;#908372

I don't have to know much to know that if the guys who did the research disavow these conclusions, there probably isn't a lot of reason to accept them.
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Old 03-22-08, 07:01 AM   #19
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Josh is totally right. I'll elaborate on the frames of reference for each claim I made in the Bicycling article very soon, but just so everyone knows most of the quotes were meant to be referenced for general cases (how you'd actually ride the bike, component, etc.). Another thing I'd like to preface everything with is that frontal area and drag are not everything! Disc and aero wheels (at certain yaw angles) can actually contribute forward thrust to the bike due to side forces involved. I'm not trying to say wear an aero helmet and ditch the wheels -- rather I'm trying to mention what should be done if you're trying to get faster on a budget. Cycling and triathlon are expensive sports and the general public should know that they aren't at a loss if they don't have a $10,000 bike!

I'll get into more detail over the next few days, but just know that I didn't make up the things I said in that article and I knew I'd need to back up a lot of the details. Dr. Kim Blair and I are extremely meticulous when we test (and when we release data). I apologize for the lack of context for a lot of the aero notes, but it was Bicycling's choice to not publish the details I included. However, I think the article points to some interesting things that haven't really been written down anywhere.

Thanks for all of your comments. -- Mark Cote, MIT Center for Sports Innovation
Like I always have, asgelle, I still strongly disagree with the use of 'disavow' to describe the above quote. The only objection that 'Joshatzipp' seems to have is that MIT only tested 0 degrees of yaw.
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Old 03-22-08, 07:11 AM   #20
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Like I always have, asgelle, I still strongly disagree with the use of 'disavow' to describe the above quote. The only objection that 'Joshatzipp' seems to have is that MIT only tested 0 degrees of yaw.
If you want to use another word, that's fine, but the point is, as MIT have agreed, they only tested at 0 yaw where helmets perform the best and wheels, the worst. As they've also agreed, 0 yaw is a very rare condition for races and a more realistic value is somewhere between 5 and 15 degrees depending on the assumptions. And finally they've agreed that at more realistic yaw angles, the decrease in drag from aero wheels is greater than the decrease from an aero helmet.

Like I've said, if the group doing the testing doesn't stand behind the Bicycling report, which by the way never speaks to relative value, only absolute performance, I don't see how anyone reading it could.
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Old 03-22-08, 08:19 AM   #21
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If you want to use another word, that's fine, but the point is, as MIT have agreed, they only tested at 0 yaw where helmets perform the best and wheels, the worst. As they've also agreed, 0 yaw is a very rare condition for races and a more realistic value is somewhere between 5 and 15 degrees depending on the assumptions. And finally they've agreed that at more realistic yaw angles, the decrease in drag from aero wheels is greater than the decrease from an aero helmet.

Like I've said, if the group doing the testing doesn't stand behind the Bicycling report, which by the way never speaks to relative value, only absolute performance, I don't see how anyone reading it could.
what's your take on the latest roues artisanles wheel testing? apparently they just fit a weighted curve to the degrees of yaw:



do you think this greatly detracts from the study? my guess would be that giving too much weight to the 0 degree measurement would likely overvalue non-aero wheels since the differences between them and aero wheels are less significant at 0 degrees.
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Old 03-22-08, 08:53 AM   #22
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http://www.zipp.com/Technology/Aerod...3/Default.aspx

Check out the graph of drag vs. yaw angle. Yes the rear wheel is drafting the frame but only in an absolute headwind.
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Old 03-22-08, 10:55 AM   #23
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^To talk about something that would require asterisks, all of Zipp's tests (to my knowledge) are done without a rider. The rear wheel will draft off of the legs as well, which hasn't been tested.
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Old 03-22-08, 11:35 AM   #24
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what's your take on the latest roues artisanles wheel testing? apparently they just fit a weighted curve to the degrees of yaw:
I haven't read the article, but in principle, I like the idea of a weighted average yaw angle for general discussions. That would collapse the data over the range of angles to a single number. The problem is how to do the weighting. As you say, the results will depend on the specific implementation so the question becomes does the weighting used reflect reality. It seems this article overweights 0 yaw, but I haven't seen any data to show it does, or to show a single weighting is robust enough to cover a wide range of riders. Until that is available, I prefer to see drag vs. yaw so readers can perform the weighting for themselves.
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Old 03-22-08, 12:48 PM   #25
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All of this is just aerodynamics though. I would think that on a flat to rolling course a disk would be faster in a TT just because it would be slightly heavier than a deep aero wheel. I a flat TT you obviously aren't having to climb and you aren't having to deal with the accelerations of a crit, you are just trying to maintain your own momentum so a extremely light wheel could be a disadvantage. The Discovery channel team actually requested the Bontrager Aeolus wheel be made slightly heavier because they believed it to be faster on flat terrain, at least thats what I read.
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